1. Field of the Invention
The present invention relates to a cladding structure of an optical fiber laser capable of allowing efficient optical pumping at a core of the optical fiber laser using the concept of quantum chaos, and more particularly, to a cladding structure of an optical fiber laser, which enables efficient pumping in an optical fiber by constructing the cladding in a non-integrable structure for causing quantum chaos so that pumping beams injected in the cladding can pass through a core of the optical fiber as much as possible.
2. Description of the Related Art
In recent years, optical fiber lasers enabling stabilized laser output of several kilowatts have been applied to a variety of industrial fields. Several principal techniques have been recently developed to obtain a laser beam of several kilowatts from an optical fiber laser. One of the techniques is a cladding design technique. Generally, a core of an optical fiber has a small diameter less than 20 μm and undergoes various problems when a laser beam is injected in the core due to the strong intensity of the laser beam. Accordingly, if a diode laser beam as an external pumping light source is injected in a cladding of an optical fiber, the diode laser beam passes through a core of the optical fiber and the laser beam is generated due to pumping at the core with erbium (Er) applied to the interior thereof. At this time, if the pumping light source passes through the core as much as possible, the pumping becomes efficient. Therefore, it is important to design the cladding through which the pumping beam passes.
As for claddings researched heretofore, a cladding is formed in a rectangular shape around an inner cylindrical core as disclosed in U.S. patent application Ser. No. 20020181512, or is formed by cutting both sides of a cylinder as disclosed in U.S. Pat. No. 6,157,763. Theses claddings are shown in FIG. 1. FIG. 1(a) shows a rectangular cladding structure, and FIG. 1(b) shows a cladding structure obtained by cutting both sides of a cylinder.
FIG. 2 shows four paths when a beam reflected on an outer boundary surface of a cladding is incident back to a core in a case where the cladding has a rectangular shape around an inner cylindrical core as in U.S. patent application Ser. No. 20020181512. It can be seen for the four paths that time required from the time when the beam is reflected on the outer boundary to the time when it reaches the core is about 0.92 seconds in FIG. 2(a), infinity in FIG. 2(b), about 12.03 seconds in FIG. 2(c), and about 217.77 seconds in FIG. 2(d). Thus, they show that in the rectangular cladding structure, it takes a great deal of time for the beam reflected on the outer boundary to reach the core or the beam cannot reach the core at all.
FIG. 3 shows a probability in connection with an optical path length that a beam will advance to a core of an optical fiber in 160 thousand arbitrary directions in the rectangular cladding around the inner cylindrical core as in U.S. patent application Ser. No. 20020181512. FIG. 3(a) shows an optical path length with respect to a probability, and FIG. 3(b) shows a logarithmic probability in which a normalized length is reduced exponentially but a probability that a beam will reach the core is lower as compared to a spiral structure. It also shows that there is a beam direction which cannot reach the core eternally due to the finite length of the optical fiber laser.
It also shows that there is a very short pumping direction as well as a long pumping direction in the rectangular cladding structure and this structure is less efficient for optical pumping.